It is known to equip various kinds of apparatus with time switches in order to put the apparatus into different conditions or modes during different parts of a predetermined period of time, for example during a 24 hour period. For instance, it is common to equip a central heating system with a time switch to cause heat outlets of the system to produce different levels of heating during different parts of the day. The time switch may comprise a plurality of terminals and means to energise the terminals in sequence. For instance, in a time switch for a central heating system, one terminal may be energised for a period of the day during which high level heating is to be provided and another terminal energised for a different period of the day during which low level heating is to be provided. A disadvantage of such a time switch is that the said two terminals, which act as output terminals of the time switch, are energised at fixed times determined by the construction of the switch, whereby the time switch must be specifically designed for a particular application.